Programming Fundamentals › Programming Basics

Global Variable

A variable visible everywhere, and why it makes code hard to reason about.

Also known as: global state, global scope, globals

A global variable is declared outside any function, so every part of the program can read it, and often change it.

counter = 0

def increment():
    global counter       # needed in Python to assign to a global
    counter += 1

Why they cause trouble

Anything can change a global, so to understand one function you have to know everything else that might touch it.

• Hidden inputs. price() gives different answers depending on a global discount set elsewhere.
• Hard to test. Tests leak state into each other. One test sets counter, and the next one starts with a surprise value.
• Order dependence. The code works only if things run in a certain order.
• Concurrency bugs. Two threads or requests updating the same global is a race condition. In a web server, a module-level variable is shared by every request.

Better options

• Pass values in as arguments and return results.
• Group state in an object that’s created and passed where needed (dependency injection).
• Use constants for fixed values (MAX_RETRIES = 3). Globals that never change are fine (constants).
• Put configuration in one place and read it from there (configuration).

Name clashes

A local variable with the same name as a global hides it inside the function (shadowing). In JavaScript, a forgotten let can create an accidental global, which is why "use strict" and modules exist.